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Sanding is not essential. It does however make it easy to fix things and give more pop to to the plaster. It makes the most difference on unburnishedd Veneziano. This done with a fine slightly worn sanding sponge or similar. You're not really sanding. Instead you're kind of compressing it a little and giving it more accent and depth of color. The more the plaster was burnished lightly, the more 'pop' you'll have when you sand. You don't have to scrub hard...just a few times in the same area. It feathers out imperfections and get rid of little bits of plaster sticking up. Sand over bullnoses and corners. Stop if your scratching the plaster. It means that you sanding implement is still too hard and gritty.
Make sure to wipe the dust when you're done. If you have areas that you're not happy with even after it dries, try wetting the area with a spray bottle (or whatever) and gently sand the area in question. The plaster will soften a little and the sanding will take off a very thin top layer of the plaster. Try this first on your sample board. Do this very gently and slowly at first. You can wipe out any 'mud' that you made on the wall with a rag. Let it dry and see what happens before you do larger sections.
If your burnished Veneziano is done perfectly and you have the whole thing polished out, you can unmask it and walk away. More often than not there will be something you might have to fix. Fixing burnished Veneziano is a little tricky. You can get a little more pop from Veneziano by sanding it with 600 grit or higher sanding paper. Do this with a large sanding block. At your own risk, you can use a mechanical sander. Don't use orbital sanders. If there's a spot that need attention try wetting the area, and wet sanding with the sand paper. If you time just right, you first take off a thin layer if the area, and as it starting to set up, the sand paper will start polishing the area and feathering it out with the sorounding plaster with no evidence of what happen. Sometimes people do whole walls in this manner to give it a different quality of sheen and 'feather' it out making it more natural. This is sometimes done when colors are combined to give a more natural combination. You can burnish the wet sanded spots with your trowel, too.
For more pop on unburnished Venziano, if sanding doesn't work, you can wet the wall and burnish it with your trowel. After it's dry, if you still need more, you sand sand the area and ot will be darker and more varied.
The physics of this whole wet sanding process is exactly the same for polishing marble and granite. This is a very touchy process, so make sure you practice. It's a good thing to know if you have to patch a burnished wall. It's sometimes the only way to fix it.
WARNING: Electric sanders and water is a dangerous combination. Do this at your own risk.